Deduce two similarities and one difference in the \(^{\text{1}}{\text{H}}\,{\text{NMR}}\) spectra of the two isomers \({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}\), a carboxylic acid, and \({\text{HCOOC}}{{\text{H}}_{\text{3}}}\), an ester. \(^{\text{1}}{\text{H}}\,{\text{NMR}}\) data are given in Table 18 of the Data Booklet.

Similarities:

Difference:

The mass spectrum of one of the two isomers above has significant peaks at mass to charge ratios of 15, 45 and 60, while the other isomer has peaks at 15, 29, 31 and 60. Analyse these fragmentation patterns in the two mass spectra in order to distinguish between the two isomers.

Similarities: [2 max]

both have two peaks;

in the same/1:3 ratio;

both have only singlet peaks;

Difference:

\({\text{C}}{{\text{H}}_3}{\text{COOH}}\) will have an absorption/chemical shift/\(\delta \) in the range 2.0–2.5, (\({\text{HCOOC}}{{\text{H}}_3}\) will not) / \({\text{HCOOC}}{{\text{H}}_3}\) will have an absorption in the range 3.8–4.1, (\({\text{C}}{{\text{H}}_3}{\text{COOH}}\) will not) / \({\text{C}}{{\text{H}}_3}{\text{COOH}}\) will have an absorption in the range 9.0–13.0, (\({\text{HCOOC}}{{\text{H}}_3}\) will not);

CH₃COOH:

peak at 45 due to \({{\text{(COOH)}}^ + }\) / \({{\text{(}}{M_{\text{r}}} - 15{\text{)}}^ + }\) due to loss of \({\text{C}}{{\text{H}}_3}\);

HCOOCH₃:

peak at 31 due to \({{\text{(OC}}{{\text{H}}_3}{\text{)}}^ + }\) / \({{\text{(}}{M_{\text{r}}} - 29{\text{)}}^ + }\) due to loss of HCO/CHO / peak at 29 due to \({{\text{(HCO)}}^ + }\)/\({{\text{(CHO)}}^ + }\) / \({{\text{(}}{M_{\text{r}}} - 31{\text{)}}^ + }\) due to loss of \({\text{OC}}{{\text{H}}_3}\);

Penalize missing + sign once only.

Brackets not required around fragments for marks.

(a) was generally answered well, the commonest errors being to include the 0.9-1.0 ppm chemical shift for the methyl group.

(b) was generally answered well, the commonest errors being omitting the charges on the ions.